This is backwards of what it should be, I think.
"switching to normal ac slows it down"
That should increase the heat load on the condenser and compressor.
Edit, I take that back, thinking out load here, if the system is on Max AC, and the flow is restricted in the expansion valve (for any reason like dirt or..), the expansion valve is letting less refrigerant flow due to less heat load, and that backs up the high pressure side till the pressure switch cuts off the clutch) and higher pressure means more work for the compressor (that part is complicated, determining the work load), so maybe not, but I was always under the impression it ran hotter load wise on normal???
I wonder if they sold you the wrong water pump, the one with a reverse direction impeller, made for I think the grand cherokee? It has happened before!!!! Look up the part number they sold you. If it was the wrong pump in the box, you need to pull it to visually determine this. old_man has mentioned this issue here before I think.
Needing 1300 rpm to keep it cool on Max AC sounds like the wrong water pump to me, but I am no expert on that, never had that problem, just read about it here and do not recall the detailed facts. You might Google search it?